OK it worked... fifth try...
I exported a new p12 development certificate from Keychain access (as described on pages 19-20).
Then I logged back into the DPS App Builder... Manage... download developer app (.ipa)... selected the new developer p12... and password....
... after getting a "server error"... tried again and it worked...
For the record I did not do anything different in each of my five attempts... but this time it worked and I was able to download the .ipa file to my desktop...